The present invention relates to manufacturing equipment and, more particularly, to wave solder machines.
Generally speaking, in a wave soldering machine a printed wiring board (PWB) is moved by conveyor on an inclined path past a fluxing station, a preheating station, and, finally, a station at which a wave of solder is caused to well upwardly and contact various portions of the PWB to be soldered. The efficiency of this wave soldering process is affected by a number of concerns, two of which are known in the an as the "contact-area" and the "dwell-time." The dwell-time represents the amount of time any given area of the PWB is in actual contact with the solder. Generally, the dwell-time is related to the contact-area and can be determined by simply dividing the length of the contact-area, which is in the direction of travel, by the speed of the conveyor belt.
The contact-area represents that portion of the PWB that is in actual contact with the solder wave, i.e., the area of the PWB that is currently covered by the solder wave. It is preferable to have a contact-area that is uniform, i.e., not uneven. A uniform contact-area is one where the dwell-time is the same for all portions of the PWB. For example, for a rectangular shape all portions of the PWB perpendicular to the direction of motion enter, and leave, the solder wave at approximately the same time. However, if the contact-area is not uniform, then some portions of the PWB are within the solder bath longer than other portions, i.e., the dwell-time changes across the PWB. For example, if the contact-area is trapezoidal (with the parallel sides in the direction of travel), one side of the PWB maintains contact with the solder longer than the opposite side. If the dwell-time is too short, the soldering process may not be completed. On the other hand if the dwell-time is too long, the PWB may become too soft and begin to sag, or solder may go up through the holes of the PWB due to strong capillary action, which could result in solder bridges forming on the component side of the board.
One method for measuring the solder wave contact-area and the dwell-time is to use a tempered glass plate that is passed over the solder wave during a "test pass." This glass plate has a set of grid lines of known dimensions. As the glass plate passes over the solder wave, the contact-area is visually observed, and approximately measured via the grid lines, by an operator of the solder wave machine. From the observed contact-area, the dwell-time is determined by dividing the length of the contact-area, i.e., that dimension in the direction of motion of the conveyor belt, by the speed of the conveyor belt.
Unfortunately, this method for measuring the contact-area is purely subjective and only yields an approximation of the contact-area. In addition, in order to observe the contact-area on the glass plate, the wave soldering machine conveyor speed must be slowed so that the operator has time to observe the glass plate.
Other methods known in the an address the determination of dwell-time but, overall, assume a uniform contact-area. For example, U.S. Pat. No. 5,023,848, issued Jun. 11, 1991 to Frey et al., addresses the problem of determining an accurate dwell-time in the presence of "drag-out." The latter effect is when solder tends to hang on to one lead, or leads, longer than other leads. In this approach, a test device includes two electrical contacts, each contact located at opposite ends of the test device. As the test device passes through a solder wave, the latter provides the electrical connectivity between this pair of electrical contacts. Eventually, the test device moves out of the solder wave, and the electrical connectivity between the pair of electrical contacts is broken. A timer measures the time duration of the electrical connectivity between the contacts. This time duration is used to represent the dwell time. Since only two electrical contacts are used, a uniform contact-area is presumed. Similarly, U.S. Pat. No. 4,180,199, issued Dec. 25, 1979 to O'Rourke et al., describes a system for measuring dwell-time and temperature profiles of a soldering operation. In particular, dwell-time is measured by the amount of time a single electrical contact is grounded by immersion in a solder bath. The use of a single electrical contact again presumes a uniform contact-area.
In that some solder wave machines provide for automatic adjustment of solder wave height presuming a uniform contact-area. For example, Model Number LGC400DR(JK) from the Sensbey Corporation adjusts the height of the solder wave by using a single probe that floats in the solder wave. Information from this single probe is then used to adjust the height of the solder wave, which, in turn, affects the size of the contact-area.